The scaling down of integrated circuits is a constant effort in order to increase the packing density and improve circuit performance. With circuits becoming smaller and faster, improving the device driving current becomes even more important. The device driving current in a metal-oxide-semiconductor (MOS) transistor is closely related to the gate's thickness, capacitance, and carrier mobility. Shortening the length of the gate, increasing the capacitance of the gate, and increasing the carrier mobility of the gate can all improve the transistor current performance. Accordingly, reducing the thickness of the gate dielectric is one of the ongoing efforts in order to shrink circuit size.
However, as the thickness of the gate dielectric is reduced, some factors, which were negligible in larger gate dielectrics, can become crucial to the proper functioning of the device. For example, in a gate dielectric with an ultra-thin gate oxide regime (where the thickness of the gate dielectric is less than or about 10 Å), the substrate/gate oxide interface, with its unwelcome electronic states and carrier traps, may dominate the electrical characteristics of the gate dielectric.
Also, in a thicker gate oxide, the thickness of the gate oxide serves to protect the substrate/gate oxide interface from unwanted dopants that might diffuse through the gate oxide contaminate the substrate/gate oxide interface. Thus, thinning the gate oxide in order to reduce the overall size also provides a smaller barrier, allowing more contamination. Excessive dopant diffused into the region near the substrate/gate oxide interface can affect the threshold voltage of the device and degrade its overall performance.
Solutions that have been used to resolve this problem either introduce nitrogen into the gate oxide or deposit a layer of silicon nitride onto the gate oxide. Introducing nitrogen into the gate oxide forms a nitridized silicon oxide layer (silicon oxynitride layer) that works as a barrier to a dopant attempting to diffuse into the semiconductor substrate. One approach to introducing nitrogen into the gate oxide is thermal nitridation of the gate oxide, where the gate silicon oxide is thermally treated in a nitrogen containing ambient such as ammonia (NH3) to form a silicon oxynitride layer.
Unfortunately, while this nitridation process works well for thicker gate oxides, the nitrogen diffusion is very difficult to control, and the nitrogen itself can diffuse into the substrate. As a result, the substrate may be contaminated as shown in FIG. 2. This nitrogen near the substrate/gate oxide interface degrades the channel mobility of the device and leads to a degradation of the drive current. Furthermore, the silicon oxynitride layer has a higher dielectric constant than silicon oxide, which causes an increase in the device saturation current (Idsat). Accordingly, this is not an effective means to prevent contamination of the substrate/gate oxide interface for an ultra-thin gate dielectric.
The other option, depositing a layer of silicon nitride above the gate oxide, adversely effects the height of the gate. While the layer of silicon nitride acts as a barrier to contaminates, depositing silicon nitride by conventional techniques such as CVD and ALD forms a very thick dielectric layer (10 to 15 Å), which would cause the gate dielectric to be larger than desired.
Because of these and other problems associated with the current methods to prevent contamination in gate oxide regions, a new method to prevent the contamination into the substrate/gate oxide interface is needed, particularly for ultra-thin gate oxide layers.